Interpretive Summary: Cercospora leaf spot (CLS), caused by the fungal pathogen Cercospora beticola, is the most important foliar disease of sugarbeet. The disease is controlled in part by timely applications of a class of fungicides called quinone outside inhibitors (QOI) or strobilurins. Past research has shown that Cercospora strains resistant to QOI fungicides have a mutation called G143A in the cytochrome b gene. We tested strains harvested from 992 fields across the Red River Valley of North Dakota and Minnesota for the G143A mutation. In total, 12 fields located in diverse locations in the RRV contained the G143A mutation. This is the first report of QOI resistance in the Red River Valley. Careful monitoring of the G143A mutation will be necessary to ensure that QOI fungicides remain efficacious for CLS management in the RRV region.

Technical Abstract: Cercospora leaf spot (CLS), caused by the fungal pathogen Cercospora beticola, is the most important foliar disease of sugarbeet. The disease is controlled in part by timely applications of quinone outside inhibitor (QOI) fungicides. However, pathogen resistance to QOI fungicides is associated with the exchange of glycine to alanine at amino acid position 143 (G143A) in the C. beticola cytochrome b gene. To assess whether QOI resistance has developed in C. beticola in the Red River Valley (RRV) of Minnesota and North Dakota, a real-time PCR procedure was used to assess whether the G143A mutation could be identified in samples harvested from 922 fields across the RRV. In total, 12 fields located in diverse locations in the RRV contained the G143A mutation, suggesting that QOI resistance arose independently at several locations in several genetic backgrounds. This is the first report of QOI resistance in the RRV. Careful monitoring of the G143A mutation will be necessary to ensure that QOI fungicides remain efficacious for CLS management in the RRV region.